What defines a RPG?

[cloud1161]

I always thought RPG's were strictly what we call JRPG's today. And games such as Diablo or Baldur's Gate or the like were action/adventure. I never thought it had anything to do with the development of the character(s) but determined by the game's battle system. It wasn't until I started actually reading gaming articles that I learned what the proper genres of games. Still, I try not to categorize so much because it is all about whether the game is fun or not.